This invention relates to an improvement in saturated, linear polyester resins and to a method for preparing such resins having improved stability.
Highly polymeric polyester resins are derived from glycols and dicarboxylic acids or ester-forming derivatives thereof by esterification or ester exchange and subsequent polycondensation. In conducting the polycondensation reaction the reactants are subjected to prolonged conditions of elevated temperatures which can produce resin products having undesirable yellow or brown coloration. This undesirable coloration is carried over to the finished product made from these resins.
In addition to the development of color, polyesters degrade on exposure to heat and to conditions that favor hydrolysis of ester compounds. It is therefore desirable to provide polyester resins with stabilization against discoloration and degradation.
Various stabilizers for synthetic polymeric materials including polyesters have been proposed. However, only a very few of them are in actual commercial use in polyester because these stabilizers are required not only to exhibit superior stabilizing effects, but also to maintain the stabilizing effects for prolonged periods of time, have good reproducibility of the stabilization effects, not affect the physical properties of the resin adversely nor to color the resin, and also be easy to produce and be available at low cost.
Previously, dialkylhydroxyphenylalkanoic acid esters of di-and tri-pentaerythritol were proposed as stabilizers for polyesters and other organic materials (U.S. Pat. No. 3,642,868).
Compounds having a hindered phenol group or an analogous group are known as stabilizers for polymers, for example U.S. Pat. No. 3,644,482, German Offenlegungschrift No. 2,150,325, 2,158,014 and 2,158,015. Furthermore, U.S. Pat. No. 3,681,431, Dutch Laid-Open Publication No. 72.09214 and Dutch Laid-Open Publication No. 72.09230 propose compounds having a hindered phenol group.
U.S. Pat. No. 3,773,723 discloses a resin stabilized against discoloration by the addition of effective amounts of dialkyl thiodipropionates.
Hindered phenolic phosphorus compounds are suggested as polyester stabilizers in U.S. Pat. No. 3,386,952 and U.S. Pat. No. 3,676,393 suggests use of phosphonates as stabilizers while Canadian Pat. No. 973,994 discloses hindered phenolic phosphite compounds.
Synergistic stabilizing combinations are disclosed in U.S. Pat. No. 3,985,705 and U.S. Pat. No. 3,987,004 while U.S. Pat. No. 3,691,131 relates to phenolic antioxidants in combination with metal hypophosphites as stabilizers and U.S. Pat. No. 3,658,705 teaches use of a halogen compound, either alone or in combination with a copper compound as a stabilizer.
U.S. Pat. No. 3,300,440 relates to thiophosphate compounds as stabilizers for polyester resins. U.S. Pat. No. 3,640,948 discloses a sulfur-containing phenol having the formula OH--.phi.--S--R wherein .phi. represents a benzene ring, the hydroxyl group positioned para or ortho to the sulfur atom and R is alkyl, aryl or phenol. U.S. Pat. No. 3,640,948 does not disclose, suggest or claim that R can be selected from the group consisting of aralkyl radicals, polycyclic radicals, bis aralkyl or bis polycyclic radicals. In addition, U.S. Pat. No. 3,640,948 does not suggest or disclose bis-sulfide products as stabilizers for polyesters.
Although the above-identified patents directed to stabilize polyesters are of major interest, certain of the proposed polyester modifiers are known to be highly toxic and/or hazardous to use on commercial scale. Moreover, it has been found that these known compounds and processes do not completely solve or mitigate the long-standing problem of producing high molecular weight polyester stabilized against deterioration and discoloration under high temperature operating conditions. Accordingly, we have carried out considerable research in this field to find 4-mercaptophenol derivatives not disclosed or suggested by the prior art to be highly effective stabilizers for polyesters.